DESCRIPTION:(From Abstract) The response of mammalian cells to ionizing radiation (IR) includes cell cycle arrest, regulation of gene transcription, activation of DNA repair and, in the event of irreparable lesions, the induction of apoptosis. The IR-induced signals which determine cell fate, that is survival or death, are largely unknown. Our work has focused on the identification of a nuclear complex, consisting in part of the Lyn and c-Abl protein tyrosine kinases, that is activated in the response to IR exposure. The findings demonstrate that the DNA-dependent protein kinase (DNA-PK) contributes to IR-induced activation of c-Abl and that both Lyn and c-Abl function in the downregulation of DNA-PK activity. The findings have also provided the basis for demonstrating that Lyn and c-Abl function as upstream effectors in determining the cellular responses to IR. The proposed studies will extend our previous work by further defining the involvement of Lyn and c-Abl in IR-induced signaling. Our hypothesis is that both Lyn and c-Abl are functional in regulating p53 and related family members in the IR response. Preliminary evidence indicates that Lyn, like c-Abl, regulates IR-induced stabilization of p53 by inhibiting the function of Mdm2. Our data also indicate that Lyn and/or c-Abl regulate p53-mediated transactivation and the p53-associated induction of topoisomerase I activity. Other studies have shown that c-Abl interacts with the p53 homolog, p73, in the apoptotic response to IR exposure. The proposed work will address the mechanistic basis for this effect and whether Lyn functions in the regulation of p73. The proposed studies will also address a model in which p63, a third member of the p53 family, is subject to IR-induced regulation by the Lyn/c-Abl complex. The specific aims are: 1) to define the role of Lyn in IR-induced activation of the p53 tumor suppressor; 2) to identify IR-induced signals that regulate topoisomerase I; 3) to further define the regulation of p73 in the IR response; and 4) to study involvement of p64 in the apoptotic response to IR exposure.